1. Field of the Invention
The invention relates to a method and a device for controlling the operating brake pressure for pressure medium brakes in a vehicle combination comprising a tractor vehicle and at least one trailer vehicle, said vehicles being connected by a conventional coupling device, according to which the driver of said tractor vehicle generates by means of a brake valve an operating pressure defining the brake pressure of said tractor and trailer vehicle, forces developing in said coupling device being measured during the course of braking and being reduced to a predetermined value.
2. Description of the Related Art
The brakes of a trailer vehicle in a vehicle combination are adjusted so as to function in a satisfying way when the trailer vehicle is fully loaded. National laws and regulations stating the lowest deceleration which the vehicle combination has to fulfil when braking must also be complied with. In practice this means that certain problems arise when the trailer vehicle is uncompletely loaded or when it is not loaded at all. The problems arising are for instance wheel locking when braking, brake overheating, unnecessary wearing of the brake lining of the trailer vehicle brakes and on the coupling device between said tractor and trailer vehicle, etc.
To overcome these problems it is suggested in SE 455 854 to provide in the coupling between said tractor and trailer vehicle a transducer for indicating pulling and pushing, respectively, of the coupling and that said transducer is operatively connected through a central unit to a valve for controlling the brake pressure to the brakes of said trailer vehicle. The control of the brake pressure is made through the service brake system of the tractor vehicle and is thus comparatively slow. Furthermore, the control can only take place as a decrease of the pressure existing in the tractor vehicle during the braking An adjustment which in certain aspects is less favourable can also take place when there is a braking on a hill, and in a non-moving vehicle if there is a signal received from the transducer while at the same time the driver presses the brake pedal.
Also other problems may arise in an incorrect distribution of the braking power between the tractor and the trailer vehicle when the vehicle combination is decelerated. In a tractor vehicle having a lower braking power than accepted by law also the braking power of the vehicle combination will be unsatisfactory and below the accepted. If the braking ability of the brakes of the tractor vehicle is too low the result is a pushing trailer vehicle, which may result in so called jack-knifing Said latter problem is suitably avoided by securing that the rear vehicle of the vehicle combination is braked first and that there is a certain amount of overcapacity in the brakes of said vehicle. Such a solution was suggested in U.S. Pat. No. 4,231,442, a device according to said patent comprising a power sensitive transducer arranged in the coupling between said tractor and trailer vehicle, an amplifier, and a switch connected to the brake pedal of said tractor vehicle. When the driver presses said braking pedal the brakes of the trailer vehicle are first activated. The pulling force thereby appearing in said coupling between the vehicles actuates through said amplifier the brakes of the tractor vehicle. The amplifier is activated only if said switch has been actuated by said brake pedal. However, there is a considerable risk in activating firstly the brakes of said trailer vehicle because if said brakes do not work properly the result may be that the vehicle combination as a whole is not decelerated sufficiently neither with respect to the actual braking requirement nor with respect to the minimal deceleration prescribed.
To avoid wearing of the tires and to decrease the risk of skidding there has also been suggested different systems comprising non-locking brakes. However, such systems do not overcome problems with an unnecessary wearing of the brake linings of the trailer vehicle brakes and on the coupling device between said tractor and trailer vehicle. Also, it is not possible to avoid problems with increasing differences in temperature between the different wheel brakes of the vehicle units. If the vehicles are provided with pressure brakes there is also a certain unnecessary waste of pressurized air during every course of braking because any excess pressure is ventilated so as to avoid wheel locking. However, non-locking brakes can be an excellent supplement to other types of brake control systems.